Advanced bearings for high-speed machining

In the world of precision machine tools, few topics receive more attention than high-speed machining. Cutting metal at higher speeds increases productivity. But to run faster without losing quality or reliability, the spindle, motor, and bearing system must provide the torque, rigidity, and accuracy for cutting while minimizing rotational friction for the tool and workpiece.

As such, the bearing design must ensure high precision and address factors that limit high-speed capabilities. For instance, any loss in bearing dynamic stability can lead to vibrations, tool chatter, wear, and costly scrap. Higher speeds generate more heat because the lubricant shear rate increases. They also boost centrifugal force of the bearing rolling elements. Both factors can alter the bearing's design preload which, in turn, can generate even more friction and heat.

Optimizing the bearing system lets machine-tool builders raise speeds and achieve “first part correct” running accuracy on a consistent basis. The latest designs improve dynamic stability for chatter-free metal removal while maintaining lower operating temperatures. They also contribute to longer machine service life.

Basic factors

There is no hard-and-fast rule for segregating “high” from “standard” speeds, but a dN value >1 million is often considered to be in the high-speed realm. A well-established means of defining bearing speed, dN = (bearing bore diameter in millimeters) X (shaft rpm). Bore diameter normalizes the speed rating because larger bearings often run slower.

Many factors come into play when specifying high-speed bearings, complicating the selection of the best bearing for a given application. Here are some important considerations.

Quality. Annular Bearing Engineering Committee (ABEC) standards, which conform to ISO standards, define tolerances for several bearing dimensions, but they make no mention of several critical parameters. Raceway uniformity, waviness and surface finish of the rolling contact surfaces, cleanliness, ball precision, preload offset limits, and internal design are just a few of the unspecified critical parameters that can affect bearing performance, spindle stiffness, and machine life. Each bearing manufacturer sets its own limits for factors not defined by ABEC/ISO.

Mounting. Typical practice calls for a set of superprecision angular-contact ball bearings at both work and drive ends of the spindle. Bearings can be mounted in duplex (two bearings), triplex, or quadruplex sets with several directional options. Precision spacers often separate bearings in a set. This increases moment rigidity and helps equalize thermal growth between the spindle housing and shaft.

Preload. Preload is the minimum thrust load established during spindle design. The load ensures minimum force to prevent the rolling elements from skidding, and also decreases axial and radial deflections imposed by operating loads. Preload has a significant impact on bearing performance in machine tools, both at low and high speeds.

The two primary preloading methods are solid (built into the bearing design) or spring loaded (using an external spring pack). Catalog preload ratings typically range from “extra light” to “heavy.” But no standard governs preload, so definitions vary among bearing manufacturers. As a general rule, bearings are preloaded no more than necessary for an application because increased preload involves a trade-off between rigidity and torque control on one hand, and higher operating temperatures on the other.

For high-speed machining, the goal is to maintain stability and minimize tooling deflection. Designers must account for complex dynamic and thermal changes that occur at high speed, and determine how they affect preload. At speeds above 750,000 dN, centrifugal forces press balls against the bearing outer ring, creating preload buildup. This can generate additional torque and heat, reducing life. At high speeds centrifugal effects alone can increase preload from light to greater-than-heavy. Timken generally recommends extra-light to medium preload for high-speed machining, but it is critical to evaluate each unique application.

Solid preload requires a precise gap between inner and outer bearing ring faces. When mounted, a locknut clamps the rings together and the faces become flush. The resulting force from collapsing this gap equals the total static bearing preload. Spindle designers must realize that tolerances vary considerably among bearing manufacturers when flush-grinding bearing faces to obtain the desired preload. Preload tolerances vary more than fourfold across the industry but, with no governing standard, this critical variation is often overlooked.

When bearings with looser preload tolerances are clamped in a spindle, final preload can vary significantly. In the worst case, final preload may decrease by a factor of three, leading to lower stiffness, tooling deflection, loss of stability, and possibly a significantly shorter service life.

The load path in spindle bearings also has a strong impact on performance. For example, as thrust load increases on a typical back-to-back-mounted bearing set, internal ball load increases in the first bearing and decreases in the second. At thrust loads roughly three times the initial preload, the second bearing will unload, causing ball skidding and a rapid increase in heat generation and wear.

At speeds above 350,000 dN, unloading can cause catastrophic bearing failure. In addition to skidding and heat, possible effects include lubricant breakdown and uneven loading on the balls. Also, there may be extreme fore and aft forces on cage ball pockets, possibly leading to cage fracture and large contact-angle changes.

Tolerance variations become even more problematical after field replacement. Preload swings and subsequent system unloading can cause tool chatter, poor service life, and premature failure.

Internal geometry. Internal geometry includes cross-raceway radius or curvature, radial play, and ball size and quantity. The interaction of these factors affects high-speed capability. For instance, high speeds require an optimized curvature for inner and outer ring raceways. This minimizes the contact area of ball and raceway. Standard high-speed designs use a slightly more-open curvature on the outer-ring raceway. Refining the ratios of inner and outer ring curvature helps ensure dynamic stability, where the outer-ring radius provides greater ball control. Any nonuniformity in the raceway surface increases contact stress and the likelihood of premature bearing failure.

Rolling Elements

Ball selection reflects a balance between load capacity, speed capability, and dynamic stability. Ball elements used for high-speed operation are typically smaller than those used at lower speeds. Smaller size allows more balls within the same envelope, providing more contact points for greater static stiffness. But smaller balls spin faster, generating more heat and lubricant shear. Ball size and quantity, therefore, involve trade-offs to optimize high-speed operation.

There is some debate within the industry regarding the impact on dynamic stability. Timken research shows that smaller balls do affect rigidity. Current small-ball designs reduce the bearing's radial clearance. This becomes a problem with a heavy interference fit on the shaft needed for high speeds, which reduces radial clearance by up to 80%. Less clearance lowers the contact angle as the inner ring pushes balls radially, and varying contact angles can reduce stability and increase ball skidding at high speeds.

The reverse is true with larger balls. They provide higher load capacity for standard operation, but at lower speeds. At high speeds, the larger mass pushes against the outer ring to produce excessive centrifugal loading, which generates heat and increases ball skidding.

Medium-size balls with optimized curvatures for high-speed operation can improve dynamic stiffness, because dynamic rigidity is proportional to nd², where n = number of balls and d = ball diameter.

Ball material and grade also play a role in terms of limiting speed, rigidity, temperature, and machining accuracy. Ceramic balls are currently the rolling element of choice for demanding applications. They are one-third the weight of their steel counterparts, meaning less centrifugal loading. Hybrid bearings with ceramic balls can run 20% faster than comparable all-steel bearings and boost stiffness by 12%.

Ball precision is measured in grades from 5 to 25, with a lower number representing higher precision. Grade 5 has much less size variation (within 0.000010 in.) and sphericity or two-point roundness (within 0.000005 in.). Grade 5 balls generate less vibration and help produce consistent runout of the workpiece. The variation among less-precise balls makes it difficult to control nonrepetitive (asynchronous) runout.

Contact Angle

Contact angle is an important consideration at high speeds. This angle measures the difference between the ball-to-raceway contact line and a plane through the ball center perpendicular to the bearing axis. Higher angles provide more axial stiffness in the direction of the applied thrust load.

The contact angle is key to optimizing the mounting arrangement of duplex spindle bearings. The most common is back-to-back mounting where angles diverge toward the shaft centerline. It resists overturning loads and provides effective operation in both float and fixed positions.

Face-to-face mounting has angles converging toward the shaft centerline. It provides equal thrust capacity in either direction, as well as radial and axial rigidity. Tandem mounting has parallel angles for double-thrust capacity in only one direction.

In other arrangements, a flush-ground tandem set of two or three bearings may mount back-to-back in combination with a single flush-ground bearing to form a triplex or quadruplex set. Triplex configurations provide high thrust capacity in one direction in a rigid mounting carrying a moderate amount of reverse thrust. The quad set provides higher thrust capacity in both directions.

Most superprecision bearing manufacturers offer contact angles ranging from 25 to 15°. Low contact angles are more rigid radially and less rigid axially, allowing more axial yield and less radial deflection than high contact angles.

In general, use low-contact-angle bearings where the operating load is principally radial. Experts recommend higher-contact-angle bearings where the application involves chiefly thrust loads and requires a high degree of axial rigidity. Thus, most machine-tool applications use low contact angles. However, when maximum axial rigidity is required in combination with heavy thrust loads, bearings with higher contact angles are preferred.

Timken recommends a 15° contact angle for high-speed machining. Higher speeds drive the rolling elements outward. The contact angle tends to increase on the inner ring (away from the bottom of the raceway) while decreasing on the outer ring (toward the bottom of the raceway). Take, for example, a bearing with a 15° design contact angle on both inner and outer rings (1:1 ratio) at low speeds. In a particular high-speed application, the inner ring angle increases to 20.3° while the outer ring angle reduces to 9.7° (a 2:1 ratio).

The ratio of inner to outer ring contact angle should be no greater than 2:1 for maximum ball control and dynamic stability. A ratio exceeding 3:1 will induce ball skidding and heat generation. A 15° contact angle combined with proper internal geometry keeps this important ratio between 1.2:1 and 1.5:1. Superior performance characteristics will result, including improved dynamic stiffness, stable operating temperatures, and the ability to accommodate interference fits required for high speeds.

Additional considerations

Cages. Cage material, tolerances, and guidance all affect bearing performance. Phenolic is currently recommended because it's light, absorbs oil for more lubricity, and maintains pocket integrity. Holding precise tolerances on ball pockets and pilot diameters ensures the cage and rolling elements spin in the same axis. The bearing industry has not agreed on the best practice for cage guidance. Some manufacturers believe the inner ring diameter grows at speed, and it should guide the cage. Timken studies show that the cage grows and should be guided by the outer ring. Timken Fafnir designs are based on outer-ring guidance with tight controls to avoid cage precession at high speeds and loads.

Shoulder construction. The design and dimensions of the raceway shoulders can affect high-speed machining. While the thrust sides of inner and outer raceways require maximum shoulder height, low shoulders on the nonthrust sides let oil flow through the bearing. Improved lubrication of contact surfaces helps reduce operating temperatures in high-speed, oil-lubricated systems.

Surface finish. Most bearing manufacturers strive for an average surface finish of 2 to 4 R_a for inner and outer raceways, depending on bearing size. Average finish, however, is not the best indicator of optimum operating smoothness. In reality, it is the peaks or asperities, not the valleys, that impact antifriction performance. Any protrusion capable of penetrating the oil film separating the rolling elements from the raceways is a source for concern. A uniform finish with low peak content provides the smooth rolling contact necessary to maintain low operating temperatures and maximize service life.

Some manufacturers leave unfinished many nonfunctional surfaces, such as counterbores, corners, and ring lands. While these noncontact areas do not directly affect bearing performance, Timken has determined that these surfaces can trap debris and impact cleanliness. The extra step of grinding nonfunctional surfaces improves overall cleanliness and justifies the effort.

Cleanliness. The impact of superprecision-bearing cleanliness on machine-tool performance is widely acknowledged, and most manufacturers assemble, test, and package their bearings in clean rooms. Points of contamination can rapidly become a source for abrasion, cracks, or spalling, as well as lubricant-film breakdown.

Timken field research indicates that hard particulate contamination and/or marginal lubrication are the most common causes of machine-tool spindle bearing failure. Contaminants, not fatigue, limit service life and performance in the inherently high-debris environment of machine tools. Contamination generally causes excessive bearing raceway wear and increases temperature and noise. The impact on the tool spindle will be unacceptable runouts and vibration.

Seals. While seals are needed to prevent contamination and lubrication problems, the challenge of high speeds proved to be beyond the capabilities of even the most advanced contact seals. Noncontact dynamic seals are now available to keep contamination out of the bearing while minimizing running torque for high-speed operation. These seals effectively retain grease inside the bearing, making it maintenance free compared to open, oil-lubricated bearings. Extended bearing service life reduces machine-tool maintenance requirements and eliminates the costs associated with expensive oil/mist lubrication systems.

Ring materials. The bearing industry is also looking at new ring materials that resist contamination-induced surface damage and improve performance. Special high-alloy steels have recently been developed with higher hardness, compressive yield strength, and dent resistance than 52100 and other high-nitrogen steels. Special processing provides a more-uniform microstructure with greater wear resistance.